As is commonly known, a traveling-wave electrode lithium niobate optical modulator (hereinafter, which will be abbreviated as an LN optical modulator) in which an optical waveguide and a traveling-wave electrode are formed on a substrate, such as lithium niobate (LiNbO3), having so-called electro-optic effect that a refractive index is varied by impressing electric field in an optical modulator which is a waveguide type optical device (hereinafter, a lithium niobate substrate is abbreviated as an LN substrate) is applied to a large-volume optical transmission system of 2.5 Gbits/s and 10 Gbits/s due to the excellent chirping characteristic.
Such an LN optical modulator is recently under review so as to be further applied to an extra-large volume optical transmission system of 40 Gbits/s, and is expected as a key device in a large-volume optical transmission system.
FIG. 7 is a top view showing a structure of an LN optical modulator according to a prior art.
In FIG. 7, reference numeral 1 is a parallelogram z-cut LN substrate, reference numerals 1a and 1b are substrate end faces which are the ends in a longitudinal direction of the substrate 1, and reference numerals 1c and 1d are substrate side faces which are the ends in a short-side direction of the substrate 1.
Further, in FIG. 7, reference numeral 2 is a Mach-Zehnder type optical waveguide formed by carrying out thermal diffusion onto Ti, reference numeral 2a is an input optical waveguide, reference numeral 2b is a Y-branch type branched optical waveguide, reference numeral 2c-1 and reference numeral 2c-2 are functional optical waveguides, reference numeral 2d is a Y-branch type coupled-wave optical waveguide, and reference numeral 2e is an output optical waveguide.
Further, in FIG. 7, reference numeral 2f is an optical input end face of the input optical waveguide 2a, reference numeral 2g is an optical output end face of the output optical waveguide 2e, reference numeral 3 is an electric signal source, reference numeral 4 is a central electrode of a traveling-wave electrode, reference numerals 5a and 5b are earth electrodes, reference numeral 6 is a glass capillary, and reference numeral 7 is a signal light monomode optical fiber.
Further, in FIG. 7, an imaginary line of reference numeral 11 denotes a package case, and reference numerals 11a and 11b denote side faces at respective top and bottom sides of the package case 11.
Note that, as not illustrated in FIG. 7, a glass capillary and a monomode optical fiber are fixed to the optical input end face 2f at the input optical waveguide 2a side in order to input a light to the input optical waveguide 2a in an actual LN optical modulator.
In this LN optical modulator in accordance with the prior art, a light wave-guided through the functional optical waveguides 2c-1 and 2c-2 is interacted with an electric signal impressed from the electric signal source 3.
Namely, as a result of carrying out phase modulation onto the electric signal impressed from the electric signal source 3 such that the phases of a light wave-guided through the functional optical waveguides 2c-1 and 2c-2 are made to be codes opposite to one another via the central conductor 4 of the traveling-wave electrode and the earth electrodes 5a and 5b, the light receives phase modulation by which the codes are made opposite to one another at the portions of the functional optical waveguides 2c-1 and 2c-2.
FIGS. 8A and 8B are views showing a state in which the signal light monomode optical fiber 7 has been fixed to the glass capillary 6, wherein FIG. 8A is a front view thereof, and FIG. 8B is a top view thereof.
FIG. 9 shows a mounting state in which the glass capillary 6 to which the signal light monomode optical fiber 7 has been fixed is fixed to the end face 1b of the z-cut LN substrate 1.
Here, in FIG. 9, reference numeral 8 is an UV cure adhesive becoming hardened by irradiating ultraviolet radiation thereto.
Note that the UV cure adhesive 8 has seeped into the end face 1b of the z-cut LN substrate 1, the glass capillary 6, and the end face of the signal light monomode optical fiber 7 as well.
As known from FIGS. 8A, 8B, and 9, with respect to the end face 1b of the z-cut LN substrate 1, the glass capillary 6, and the end face of the signal light monomode optical fiber 7, a light from the output optical waveguide 2e is reflected at the substrate end face 1b (to be exact, the optical output end face 2g formed at the substrate end face 1b). In order to avoid the reflected light from being coupled to the output optical waveguide 2e again, i.e., in order to remove the reflected return light, the substrate end face 1b is cut at a slant.
Hereinafter, in order to simplify the description, as shown in FIG. 9, it is suppose that the output optical waveguide 2e is in parallel with the side faces 1c and 1d of the z-cut LN substrate 1 (or the package case side faces 11a and 11b).
FIG. 10 shows the situation that a light is refracted at the substrate end face 1b of the z-cut LN substrate 1 of FIG. 7 (to be exact, it is the optical output end face 2g formed at the substrate 1b. However, to be simple, it will be inscribed as the substrate end face 1b hereinafter).
It is assumed that the output optical waveguide 2e is in parallel with the substrate side faces 1c and 1d (or the package case side faces 11a and 11b). Thus, the end face 1b of the z-cut LN substrate 1 is inclined at only θ0 to the perpendicular line with respect to the side faces 1c and 1d of the z-cut LN substrate 1 (or the perpendicular line with respect to the package case side faces 11a and 11b).
Note that, in other words, θ0 is an angle formed by a perpendicular line 10 with respect to the end face 1b of the z-cut LN substrate 1 and a light propagating through the output optical waveguide 2e. 
Here, nLN is an equivalent refractive index of the output optical waveguide 2e. 
Note that a refractive index of the UV cure adhesive 8 and an equivalent refractive index of the signal light monomode light fiber 7 are expressed as n on the assumption that those are equivalent to one another.
The light propagating through the output optical waveguide 2e is refracted by Snell's law at the end face 1b of the z-cut LN substrate 1 (as described above, to be exact, although it is the optical output end face 2g formed at the substrate end face 1b, in order to simplify, it will be described as the substrate end face 1b hereinafter)
Here, suppose that an angle formed by the light propagating while being refracted at the end face of the z-cut LN substrate 1 is Θ.
In FIG. 10, Δθ(=Θ−θ0) is an angle formed by the light refracted at the substrate end face 1b and the line parallel to the side faces 1c and 1d of the z-cut LN substrate 1 (or the package case side faces 11a and 11b).
As can be understood from FIG. 10, the output optical waveguide 2e is designed so as to be parallel to the side faces 1c and 1d of the z-cut LN substrate 1 (or the package case side faces 11a and 11b) in general. For this reason, Δθ of the light refracted on the basis of Snell's law at the substrate end face 1b inclined at an angle of θ to the perpendicular line with respect to the side faces 1c and 1d of the z-cut LN substrate 1 or the perpendicular line with respect to the package case side faces 11a and 11b is not made 0.
Namely, the light refracted at the substrate end face 1b propagates at an angle of Δθ to the line parallel to the side faces 1c and 1d of the z-cut LN substrate 1 (or the package case side faces 11a and 11b).
As is well known, a coupling efficiency η when a Gaussian beam whose wavelength is λ and spot size is w is coupled to an angular declination at an angle of Δθ is given by the following formula (refer to “Basis and Application of Optical Coupling System for Optical Devices” by Kenji Kohno, Second Edition, Gendai Kougakusha, June 1998, p 45, p. 168.).η=exp(−(π·w·Δθ/λ)2)   (1)
Namely, when the signal light monomode optical fiber 7 is installed so as to be parallel to the side faces 1c and 1d of the z-cut LN substrate 1 (or the package case side faces 11a and 11b), a slant declination at Δθ is brought about between the light refracted at the substrate end face 1b and the optical axis of the signal light monomode optical fiber. As a result, a coupling loss expressed by formula (1) is generated when the refracted light is coupled to the signal light monomode optical fiber 7.
Accordingly, in order to suppress an increase in loss of the light due to the angular declination, as shown in FIG. 9, it is necessary to fix the signal light monomode optical fiber 7 at a slant to the package case side faces 11a and 11b. 
FIG. 11 shows a top view of a state in which the signal light monomode optical fiber 7 and the glass capillary 6 in FIG. 7 are mounted in the package case 11.
In FIG. 11, reference numeral 11 is a package case, 12 is a fiber covering material, 13 is a solder material for airtight sealing, 14 is a tube portion of the package case 11, and 15 is an adhesive fixing the fiber covering material 12 of the signal light monomode optical fiber 7 to the tube portion 14 of the package case 11.
As described above, the signal light monomode optical fiber 7 is fixed to the end face 1b of the z-cut LN substrate 1 at a slant of large angle. Therefore, the signal light monomode optical fiber 7 and the fiber covering material 12 are inclined at a large angle in the tube portion 14 of the package case 11.
By the way, when the signal light monomode optical fiber 7 and the glass capillary 6 are fixed to the end face 1b of the z-cut LN substrate 1, it is necessary to adjust the position of the signal light monomode optical fiber 7 in a direction perpendicular to the optical axis and in the optical axis direction in order to bring the output optical waveguide 2e and the optical axis of the signal light monomode optical fiber 7 in line.
However, as described above, in the prior art shown in FIG. 11, the signal light monomode optical fiber 7 and the fiber covering material 12 are inclined in the tube portion 14 of the package case 11 as well.
Then, the side faces 1c and 1d of the z-cut LN substrate 1 or the package case side faces 11a and 11b are made to be reference lines for mounting when the signal light monomode optical fiber 7 is mounted. For this reason, in the first place, it is technically difficult to adjust and mount the signal light monomode optical fiber 7 so as to be positioned at a slant of a large angle to those reference lines.
Further, the inclination of the signal light monomode optical fiber 7 is large. Thus, in order to ensure a sufficient margin for positioning, a diameter D1 of the hole through which the signal light monomode optical fiber 7 passes is made large to be about 2 mm, and the inside diameter D2 of the tube portion 14 is made large to be about 5 mm.
Accordingly, it is necessary to use the solder material 13 for airtight sealing in large quantity. Because the tube portion 14 is kept at a high temperature of 200° C. or more for several tens of seconds in order to melt the solder material 13, there is a problem that the fiber covering material which is weak to high heat changes in quality.
As described above, in the prior art, a light output from the LN substrate end face is output at a slant to the direction of the side faces of the LN substrate (or the side faces of the package case), and therefore, the signal light monomode optical fiber as well is fixed at a slant of a large angle to the package case.
As a result, in the prior art as described above, it is difficult to position the signal light monomode optical fiber, or it is impossible to sufficiently carry out positioning. Moreover, when an attempt is made to carry out positioning sufficiently, the inside diameter of the tube portion of the package case is made large, which brings about the problem that it is difficult to airtight-seal without the fiber covering material being damaged.
Therefore, the development of a waveguide type optical device having a structure in which it is possible to easily carry out mounting including positioning work and fixing work of the signal light monomode optical fiber so as to include making an inclined angle of the monomode optical fiber small has been desired.